The most important factor, and the one on which the success or failure of a DC motor and commutator-type AC motor depends, is commutation. In simple terms, commutation is achieved by the rotation of the commutator in contact with fixed brushes resulting in the movement of current into the coils of the motor armature. Satisfactory commutation means operating under reasonable conditions without excessive sparking, burning of the commutator brushes, or other conditions requiring excessive maintenance.
The conventional brush spring design operates over a range of pressures. In the conventional design, brush pressure against the commutator is at a maximum at the beginning of brush life. If this maximum pressure is too strong, sparking in the motor and/or burning of the brushes may occur accelerating mechanical brush wear. As brush material is worn away and transferred to the commutator by friction between the commutator and the brushes, the initial contact points are worn away and the brush begins to conform to the curvature of the commutator surface. As the brush continues to wear down, however, the pressure applied by the brush on the commutator decreases. At low pressures, the motor may experience electrical loses or intermittent contact due to the poor contact between the brush and commutator. As a result of this decreasing pressure, electrical brush wear is prematurely accelerated.